1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a one-time programmable (OTP) unit cell of a nonvolatile memory device and a method for fabricating the same.
2. Description of Related Art
The OTP unit cell is used for a memory repair in a volatile or a nonvolatile memory device such as dynamic random access memory (DRAM), electrically erasable programmable read-only memory (EEPROM) and Flash. The OTP unit cell is also used for trimming an internal operation voltage or a frequency in a mixed-signal chip where an analog chip and a digital chip exist together.
In general, an OTP unit cell includes an anti-fuse consisting of a metal-oxide-semiconductor field effect transistor (MOSFET, hereafter, referred to as “MOS transistor”) and one or more MOS transistors. The OTP unit cell is formed with a single type or an array type and used for repair or trimming.
FIG. 1 illustrates a cross-sectional view of a typical anti-fuse of an OTP unit cell. For convenience in description, a certain portion including the typical anti-fuse of the OTP unit cell is illustrated and other transistors consisting of other OTP unit cells are not illustrated.
Referring to FIG. 1, the typical anti-fuse of the OTP unit cell includes a substrate 100, a gate electrode 105 formed over the substrate 100 and a junction region 106, such as a source region and a drain region, formed in a portion of the substrate 100 exposed by sidewalls of the gate electrode 105. Furthermore, the typical anti-fuse further includes a gate dielectric layer 104 having a comparatively thin thickness formed between the gate electrode 105 and the substrate 100.
Writing operation of the typical OTP unit cell with the above described structure will be described hereafter.
The junction region 106 and a pick-up region 107 are interconnected and also connected with a VSS. The pick-up region 107 is used to apply a bias to a well 101. A writing voltage VWR is applied to the gate electrode 105 through a metal interconnection layer 108. Thus, a high field effect occurs between the gate electrode 105 and the substrate 100 and causes a breakdown of the gate dielectric layer 104. Therefore, the gate electrode 105 and the substrate 100 have a short-circuit.
However, the gate dielectric layer 104 of the typical anti-fuse of the OTP unit cell has a limitation in that it is not stably broken-down by the applied writing voltage.
During the writing operation, the writing voltage VWR transferred to the gate electrode 105 through the metal interconnection layer 108 has a descent voltage by a sheet resistance RS caused by the metal interconnection layer 108 and the descent writing voltage VWR is applied to the gate electrode 105. Therefore, since the field-effect between the gate electrode 105 and the substrate 100 is decreased as much as the descent voltage, the gate dielectric layer 104 is not stably broken-down.
Furthermore, during the initial period of the writing operation, the gate dielectric layer 104 is partially broken-down and the gate electrode 105 and the substrate 100 are partially short-circuited. In this case, there is a problem in that, a high field effect is not formed continuously between the gate electrode 105 and the substrate 100 due to a leakage current between the well 101 and a channel stop region 102. The partial break-down represents a state that the gate dielectric layer 104 is not broken-down as much as target level.
For example, when the well 101 is a P-well as shown in FIG. 1, the channel stop region 102 is doped with N-type impurity. Thus, a forward diode is formed between the well 101 and the channel stop region 102. Therefore, when the gate electrode 105 and the substrate 100 are partially shorted due to partial break-down of the gate dielectric layer 104, the forward diode operates and a leakage current is caused.
When a leakage current is caused between the well 101 and the channel stop region 102, it is hard to cause a stable breakdown of the gate dielectric layer 104 as much as a target level. Thus, the device malfunctions since a sensing margin of data is decreased during a read operation. Such malfunction decreases reliability of the reading operation of the OTP unit cell.